Leek plants (Allium porrum) were grown on partially sterilized soil either inoculated (M) or not (NM) with the vesicular-arbuscular mycorrhizal fungus, Glomus mosseae. They were pulse-fed with 14CO2 in an apparatus which allowed CO2 subsequently respired either by the shoots or by the roots plus soil to be separately monitored. There were three experiments. In two, plants were harvested 48 h after labelling and in the third after 214 h. At harvest, the distribution of 14C between shoot, root, soil organic matter and root washings was measured.
Similar growth curves for M and NM plants were obtained by supplying extra phosphorus to the latter, so that C distributions for both treatments could be compared directly. In all three experiments, about 7 % more of the total fixed C was translocated from shoot to root in M plants compared to NM plants. In the third experiment, this extra translocate could be accounted for by increased root respiration plus increased loss of C to the soil but, despite this drain, M and NM plants had equal rates of C assimilation per unit of leaf area. However, shoots of M plants had a lower content of dry matter and hence higher assimilation rates expressed on a dry matter basis.
Increased hydration is suggested as a mechanism whereby leaf area and hence C assimilation increases in mycorrhizal plants and which offsets the effects of the drain imposed by the mycorrhizas.